The invention relates to a trimming method for PLL circuits which operate on the principle of two-point modulation. The invention relates, in particular, to a method for amplitude trimming in transceivers for mobile radio systems having a PLL circuit, which operates on the principle of two-point modulation, in the transmitter and having a receiver which operates on the limiter/discriminator principle.
A low-complexity implementation of a transmitter concept for transceivers in mobile radio systems is provided by transmitters having a modulator which operates on the principle, which is known per se, of two-point modulation, in which it is possible to modulate a PLL (phase locked loop) circuit with signals whose bandwidth is greater than the PLL bandwidth, so that the PLL circuit has a transmission response which is independent of frequency.
FIG. 3 shows, in simplified form, a prior art PLL circuit, which operates on the principle of two-point modulation, for such a transmitter. The circuit has a phase frequency detector (PFD) 1, a charge pump (CP) 2, a loop filter (LF) 3 and a voltage controlled oscillator (VCO) 5 in the forward path. There is provided a frequency divider 6 with divisor N in the feedback path of the modulator.
In order to produce the two-point modulation, analog modulation is introduced into the PLL circuit, which has been stabilized at the channel mid-frequency before the actual transmission process, at a summation point 4 which is located upstream of the voltage controlled oscillator 5 in the forward path and represents a high-pass point (analog modulation which is fed in at this point acts on the output with high-pass filtering provided by the closed control loop), and digital modulation is introduced into the PLL circuit, which has been stabilized at the channel mid-frequency before the actual transmission process, in the feedback path at the frequency divider 6, which represents a low-pass point. The two modulation signals are then superimposed at the output of the PLL circuit so as to produce the desired frequency-independent response.
In this type of transmitter concept, the PLL control loop remains closed. Owing to the requirements for the noise response, the bandwidth of the PLL control loop is furthermore designed to be narrower than would be necessary for transmitting modulated data. Thus, in addition to the pure digital modulation, the analog modulation is used to compensate for the restricted bandwidth in which case, in addition to the analog and digital modulation being synchronized in phase, it is of major importance that the amplitudes of these two modulation signals match.
Owing to the production tolerances in the components that are used for analog modulation, with regard, for example, to the modulation gradient, the production of the modulation voltage and the like, it is necessary to carry out amplitude trimming between the analog modulation and the digital modulation after production. If, furthermore, it is also necessary to take into account influences caused by temperature changes, this amplitude trimming must be carried out before every transmission process.
One prior art trimming method is to introduce the two modulations, to supply the output signal from the PLL circuit to an external measurement receiver, to demodulate it there, and to carry out appropriate amplitude trimming. Since the voltage controlled oscillator 5 has a non-linear frequency response as a function of the voltage, such amplitude trimming must, however, be carried out for each of a large number of channels, which leads to a long measurement duration and, furthermore, requires that the trimming information be stored in a memory. Furthermore, it is not possible to take account of influences resulting from temperature changes in this way.
A further known trimming method includes the reception and demodulation by the receiver section of the transmitter/receiver. However, a complete second PLL circuit is required in the receiver for this purpose, which, in addition to considerably greater circuit complexity and greater costs associated with this complexity, makes it necessary when using a heterodyne receiver to set this receiver to a frequency which corresponds to the difference between the transmission frequency and the intermediate frequency.
The prior art trimming methods thus have the disadvantage that they are associated on the one hand with a long measurement duration and on the other hand with a high level of equipment and circuitry complexity, with correspondingly high costs.
It is accordingly an object of the invention to provide a trimming method for a transceiver with two-point modulation which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows rapid amplitude trimming with little complexity, and which makes it possible to take into account temperature influences.
With the foregoing and other objects in view there is provided, in accordance with the invention, a trimming method for a transmitter/receiver having a phase locked loop circuit operating on a two-point modulation principle. The method comprises the following steps:
selecting an amplitude of an analog modulation signal based on a modulation shift of a defined digital modulation signal;
applying a predetermined data sequence of the analog modulation signal;
determining a modulation shift of the analog modulation signal at an output of the receiver; and
correcting the amplitude of the analog modulation signal to match a difference between the modulation shift of the digital modulation signal and the modulation shift of the analog modulation signal.
In other words, the objects are achieved, according to the invention, with a trimming method for a transmitter/receiver having a PLL circuit which operates on the principle of two-point modulation, as follows: selection of the amplitude of an analog modulation signal on the basis of the modulation shift of a defined digital modulation signal; application of a predetermined data sequence of the analog modulation signal; determination of the modulation shift of the analog modulation signal at an output of the receiver; and correction of the amplitude of the analog modulation signal to match the difference between the modulation shift of the digital modulation signal and the determined modulation shift of the analog modulation signal.
Before carrying out the trimming process, the PLL circuit is set to a channel mid-frequency before a transmission process, in order to produce a steady state.
The digital modulation signal is preferably deactivated during the trimming process, in order to prevent the analog modulation being regulated out when selecting an initial amplitude for the analog modulation signal.
The predetermined data sequence of the analog modulation signal is furthermore preferably applied to a predetermined high-pass point in the forward path of the PLL circuit, and the digital modulation signal is applied to a predetermined low-pass point in the feed-back path of the PLL circuit, thus resulting in the PLL circuit having an overall transfer function which is advantageous for the response of the arrangement.
The digital modulation signal is in this case applied in some suitable manner to a first frequency divider.
If the output signal from the PLL circuit is passed to a second frequency divider, which is located in a signal path which branches off from the feedback path, is divided in the second frequency divider and is then supplied as one of the input signals to the receiver, there is advantageously no need for a second PLL circuit in the receiver.
The divisor of the second frequency divider is in this case preferably selected such that the output frequency from the second frequency divider corresponds to the intermediate frequency of the receiver.
Alternatively, an integer value may be selected as the divisor of the second frequency divider such that the output frequency from the second frequency divider is essentially close to the intermediate frequency of the receiver, thus resulting in an additional degree of freedom for the selection of the divisor for the second frequency divider, taking into account the actual frequency band of the receiver.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a trimming method for a transceiver using two-point modulation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.